Andrei Costan, Jennifer Bufford, Will Godsoe and Philip E. Hulme
The Bio-Protection Research Centre, PO Box 84, Lincoln University, Christchurch, New Zealand
Release from natural enemies in the introduced range has often been implicated as one reason for the success of biological invasions, particularly as a result of the evolution of increased competitive ability (EICA). Formal tests of the EICA hypothesis require evidence of shifts in herbivory, competitive ability and chemical defence between native and introduced provenances of the target species. However, many studies only assess competitive ability and are thus not complete tests of EICA. We have previously found that Rumex obtusifolisus, R. crispus and R. conglomeratus (Polygonaceae) experienced between seven times lower leaf and 22 times lower root herbivory in the introduced range (New Zealand) than their native range (United Kingdom). We therefore examined the consequences of reduced herbivore pressure on the intraspecific competitive ability and plant chemistry of provenances from the native and introduced ranges. A large-scale glasshouse experiment revealed that despite plant biomass of each species being markedly reduced, by as much as 50%, when in competition with a congener from either the same or different provenance, there was no difference in competitive ability between native and introduced provenances for any of the three species. Such a result would be consistent with plants failing to re-allocate anti-herbivore defensive compounds to growth. Rumex spp. produce a number of compounds that may have anti-herbivore properties including oxalates, phenols and tannins. We assayed these compounds from plants derived from the same populations as used in the competition experiments. Although oxalates, phenols and tannins were found in high concentrations, in particular oxalic acid (ranging between 12 and 23 g/100g dry weight) and thus potentially represent a significant metabolic cost, there were no differences between native and introduced provenances. These results provide convincing evidence to counter EICA being a mechanism for the success of Rumex spp. in New Zealand. Despite the escape from specialist herbivores that have a major impact on the populations of the species in their native range, Rumex spp. did not respond by shifting defensive chemistry towards increased growth. This points to limited flexibility in metabolic pathways as a result of compounds playing multiple function in Rumex spp and/or selection for these compounds being maintained by generalist herbivores, particularly grazing vertebrates that are widespread in both the native and introduced ranges.